Photolithography
This invention relates to the field of microlithography for the manufacture of integrated circuits, magnetic devices, and other microdevices such as micromachines. In this field the final product is manufactured in sequential manner in which various patterns are first produced in a "resist" material with each pattern subsequently defining a product attribute. The "resist" materials generally polymer compositions, are sensitive to light or other forms of radiation. The patterns are formed in the resist by exposing different regions of the resist material to different radiation doses. In the bright (high dose) regions, chemical changes take place in the resist that cause it to dissolve more easily (for positive resists) or less easily (negative resists) than in dim (low dose) regions. The bright and dim regions are formed using an exposure tool which generally transfers corresponding features to the resist from a mask or reticle. The masks or reticles are formed from mask blanks, which are plates of quartz coated with an opaque material such as chrome. The chrome is etched away in a pattern to form the mask. The radiation employed may be (but is not limited to) ultraviolet light and x-rays, and the regions of the mask that are opaque and transparent form a pattern of bright and dark when illuminated uniformly. In the most common implementation of this technology, a projection lens forms an image of the mask pattern in the resist film on a planar substrate. That image comprises the high and low dose regions that produce the resist pattern. When some form of light is employed in this process, it is called photolithography.
Wavefront Engineering
The patterns formed in the resist are not identical to those on the mask, and the methods of obtaining the pattern desired for the ultimate manufactured device in spite of deficiencies in the microlithography process is called "wavefront engineering." Among the various devices used for this purpose are phase shifting masks (PSM)s-which create desired dark regions though interference. Phase shift masks were first published by the inventor of the present invention in a paper entitled "Improving resolution in photolithography with a phase shifting mask," M. D. Levenson, N. S. Viswanathan, and R. A. Simpson, IEEE Trans. Electron Devices ED-29, 1828-1836 (1982). Since that time, there have been hundreds of patents and thousands of papers issued containing the phrase "phase shift mask". However, the technology is presently used only in applications such as memory chips and microprocessor chips. The inventor of the present invention has realized that the design and construction of the required lithography masks is so expensive that the investment required can not be returned on a few hundred or thousands of wafers. The present invention shows a way to produce phase shift masks in a cost-effective way, so that the same phase shift mask substrate design may be used with many different device designs by trading off maximum density of features on a device with cost for low volume runs.
There are presently two types of PSMs in use: weak-PSMs such as the Attenuated-PSM and strong-PSMs such as the Alternating-Aperture-PSM. These two differ in that the weak-PSMs have only one type of bright feature, while the strong-PSMs contain two types of bright features identical except for the optical phase, which differs by .about.180.degree.. See, for example, M. Shibuya, Japanese Patent Showa 62-50811, M. D. Levenson et. al. IEEE Trans. Elect. Dev. ED-29, 1828-1836 (1982), and M. D. Levenson, Microlithograpy World 6-12 (March/April 1992).
Alternating Aperture PSMs
FIGS. 1(A-C) shows plan, side elevation (along cut A), and end elevation (along cut B) views of the result of steps in construction of an alternating aperture PSM as currently implemented commercially. A substrate 10 is made of a material such as a fused quartz plate or other stable material which must be transparent to the light used in the photolithography for a transmission mask. The substrate 10 coated with an opaque ("chrome") film 12 in which openings 14 and 16 have been opened by normal photoresist application, exposure, and development, followed by chrome etch to form a conventional chrome-on-glass (COG) photomask. After stripping the original photoresist, the photomask is then recoated with a resist film (hatched areas 22 of FIGS. 2(A-C)) and apertures 20 are opened in the resist film at the locations of apertures 14 which will be phase-shifted. The openings in this second resist film are larger than those in the underlying chrome to accommodate possible mis-registration. The photomask is then etched and the chrome 12 exposed in the resist openings is used as a mask to etch the underlying substrate 10 to a depth d below the original surface to make depressions 24 as shown in the view of FIGS. 2(A-C) taken after etching of the substrate 10. The depth d of the features 24 etched in the substrate 10 is carefully chosen on the basis of the wavelength of the light to be used in the photolithography and the difference in the index of refraction of the material of the substrate and the ambient atmosphere in which the phase shift mask is used.
A plan view of the etched substrate 10 of FIG. 2A with the chrome removed is shown in FIG. 3 where the hatched areas 32 correspond to the etched phase-shifted apertures 24 in FIG. 2. The substrate 10 etched and patterned as shown in FIG. 2 defines a small part of a phase shift mask used to produce patterns in a photoresist. The difference in phase velocities of radiation in the air and in the substrate 10 material produces a 180.degree. phase shift in the light passing through regions 16 and regions 20 of the phase shift mask shown in FIGS. 2(A-C), (with photoresist removed), which result in destructive interference and which cancels the light amplitude in the region between regions 16 and 24. The term "alternating aperture-PSM" refers to the fact that the transparent apertures on opposite sides of a dark line have alternate (0.degree.-180.degree.) phases. The alternation in phase between otherwise identical apertures doubles the period of the optical amplitude pattern which corresponds to a given intensity pattern. Thus, a given projection exposure tool can create resist patterns smaller by a factor of 2 (or more) when using an alternating aperture PSM, and dramatically increase the depth of focus. In particular, robust isolated dark lines characteristic of transistor gates can be made 3.times. thinner, dramatically increasing circuit speed
FIG. 4 shows the pattern of exposed photoresist 44 and unexposed photoresist 42 resulting when light passes through the regions 16 and 20 of the mask of FIG. 2. The pattern shown in FIG. 4 is typically 4 or 5 times smaller than the pattern of the mask shown in FIG. 2A. The width 40 of the exposed areas of the photoresist is typically greater than the wavelength of the light used for exposure.
In known art, the pattern of phase-shifting is different from that of the open (non phase shifted) apertures and must be customized for each mask of each product. Such masks require multiple customized patterns to be written on each mask substrate.
In order to ensure that the two types of aperture perform identically in an optical sense, except for the phase-shift, the substrate of the prior art may or may not be etched back laterally under the opaque film as shown in FIG. 5, thus possibly leaving the opaque film unsupported at the edge 50. The non phase shift apertures 52 and 54 and the phase shift apertures 58 are noted. The trenches 56 and 58 etched in the substrate beneath the apertures are necessarily formed after the apertures are etched in the opaque layer, which is a high-cost process. The requirement to form a second custom pattern--by a process that can result in uncorrectable defects--significantly raises the cost of producing alternating aperture-PSMs. The design cost is also larger than for conventional masks as at least two mask patterns (one for brightness and one for phase) must be designed and checked for each circuit level.
U.S. Pat. No. 5,807,649 teaches a double exposure system for exposing a photoresist using a phase shift mask and with a second mask to expose unwanted dark areas left by the phase shift mask.
U.S. Pat. No. 5,620,816 teaches a double exposure system where a chromeless phase-edge shift mask is used to expose all of the photoresist except on lines running in rows and/or columns, and then a customized mask is used to expose unwanted portions of the lines and/or columns. The chromeless phase shift mask method is deficient in that the width of the unexposed lines can not be controlled, and that the unexposed lines are not totally unexposed as is shown by the 4.69% (of presumed flood exposure) shown. Chromeless masks typically have a minimum exposure in the phase shift areas of 10% or more. This problem is worsened as the masks and optics accumulate dirt in the real world of manufacturing. The chromeless mask is also deficient in that defects in the etched and non etched areas generally may not be repaired. The chromeless mask is deficient in that the exposure region where two or four chips meet on the wafer is typically overexposed by a factor two or four and the resist "blows out" for a region about these areas. The chromeless mask of the above patent is deficient in that the crossing lines left unexposed may degrade device performance. Finally, the chromeless mask of the U.S. Pat. No. 5,620,816 may only be used to define a set of lines, and not of features typically needed in lithographic patterns. The phase shift mask of the above identified patent is suited to double expose a series of perpendicular lines using the same or a similar mask rotated, and then to further expose some of the unexposed array of points to make a contact pattern.
PSM Design
Various Electronic Design Automation (EDA) tools are known for preparing the patterns used in conventional and phase-shifting masks. In addition, OPC tools alter those patterns to account for the realities of the exposure systems. It is also known that the pattern of apertures on the phase-shifting mask need not correspond closely to the ultimate circuit pattern, at least not when a conventional block-out mask is employed for a second exposure on the resist film in concert with a first exposure made using an alternating-aperture PSM. Such second exposures erase anomalies due to phase-conflicts. Numerical Technologies, Inc., in U.S. Pat. No. 5,858,580, in particular, has demonstrated the In-Phase design system which employs a block-out mask similar in geometry to the ultimate circuit feature along with an altemating-aperture PSM composed of pairs of small apertures (shifters), one of which has 0.degree. phase, while the other has 180.degree.--which define the narrowest dark features between them.
ASIC Applications of PSMs
Application-specific integrated circuits (ASICs) are typically made in too small production runs to support the extra cost of a PSM that requires two patterning steps. Other methods of wavefront engineering have been suggested to help shrink the circuit dimensions for these devices. However, none of them permit such narrow gate-like features as the alternating aperture-PSM. That may mean that the speed of low-volume ASICs will soon fall below that of DRAMs and mass-produced microprocessors, which can support the cost of advanced mask technology.
However, many ASIC chips have relatively low density, since the overall size of the chip is constrained by the need for sufficient input/output pads. The present invention is a method of patterning alternating-aperture phase-shifting masks for low density circuits which realizes the full advantage of previous PSM techniques, while dramatically reducing costs. It is especially suitable for ASICs, but may also be useful for larger-volume circuits.